“Backache” affects about 60% of the population aged over 65/70 years, and approximately one third of sufferers have to undergo spinal surgery.
Although the etiology of backache remains partially unexplained, reliable clinical data trace the cause to a slow degenerative process in the intervertebral disk. Such a process may be due to various causes:                altered metabolic activity;        reduced supply of nutrients;        decreased cell viability;        mainly age-related cell senescence.        
For the surgical treatment of said pathology, two procedures are currently in use: removal of the disk (diskectomy) and the fusion of the two adjacent vertebrae (spinal fusion).
Vertebrae can be fused together by a special surgical technique that prevents any movement between them.
This type of intervertebral fusion is also performed in other pathologies, namely:                breakage of one or more vertebrae;        to correct spinal deformities such as scoliosis;        Following removal of an intervertebral disk;        Infections and/or tumours that may cause degradation of the vertebral body;        To treat vertebral instability where the vertebrae are prone to slide on one another. This condition is called spondylolisthesis and may cause compression of the nerve roots, so that, besides pain, there may be impaired movement of the upper and lower limbs.        
There are various surgical techniques and methods of fusing two adjacent vertebrae but they all involve the introduction/application of a bone graft, generally between two vertebrae, or bone substitutes of various kinds of different shapes and sizes, such as pins, plugs or small plates fitted in the intervertebral spaces to prevent collapse and assist fusion.
Said measures are aimed to:                maintain correct alignment between the two vertebrae;        maintain and reconstruct the intervertebral space;        consolidate fusion;        eliminate pain caused by compression of the nerve root due to slipping or herniation of the disk.        
It is known that spinal fusion may also require additional fixing at the back of the two vertebrae, using rigid metal instruments of various kinds and sizes, such as screws, plugs, pins, plates, intervertebral connectors in various materials, with or without a screwable thread (for example, titanium), to prevent the vertebrae from slipping on one another with consequent compression and loss of alignment, while fusion is established.
These devices do not undergo resorption so they generally remain at the site of implantation until they are surgically removed once fusion is complete.
For many years, the material used for bone grafts was of bovine origin, or it was constituted by fragments from the tibia, fibula, femur or iliac crest of autologous or heterologous origin, with a fusion success rate ranging between 63 and 95%.
The fusion process is similar to that which occurs following bone fracture and is not visible on X-ray till six weeks after surgery.
The vertebrae can be fused in the intervertebral space and/or to the front between the two adjacent vertebral bodies and/or to the back between adjacent transverse processes, laminae, or between other posterior elements of the vertebrae, according to the pathology that the surgery in question is intended to treat.
As we have already said, solid fusion is generally achieved by grafting autologous or allogenic bone, both having specific advantages and disadvantages.
Indeed, in the case of autogenous bone, it may prove difficult to find a quantity that is sufficient for the purpose of the graft. Allogenic bone, on the other hand, is sure to have less osteoinductive activities.
These difficulties have led to the study and development of bone substitutes of synthetic, semi-synthetic and bioengineering origin, that is, to the construction of two- and three-dimensional osteoconductive matrices able to induce the migration of cells within their structure for the subsequent formation of bone.
Research was then focused on the study of the physiological mechanisms involved in bone repair and regeneration.
Bone is constituted by cells immersed in an extracellular matrix, 30-35% of the dry weight of which is represented by organic matrix (formed by fibres of collagen and glycosaminoglycans including hyaluronic acid), and inorganic substances (including calcium phosphate, calcium and magnesium fluoride) deposited among the collagen fibrils during the mineralisation phase.
Bone metabolism is regulated by hormones and growth factors mainly released by platelets, macrophages, fibroblasts or other types of cell, and chiefly includes, for example, proteins such as BMP, TGF, PDGF, FGF, EGF, IGF and VEGF that can have both a osteoinductive and angiogenic effect on the mesenchymal cells of bone marrow.
Special three-dimensional matrices have been designed and developed in various forms with different types of polymer, such as poly-L-lactic acid, poly-glycolic acid and poly-lactic-co-glycolic acid, for the formation of scaffolds (possibly containing also trophic and/or osteoinductive factors) that can favour the migration of bone progenitor cells within their structure for the regeneration/formation of new bone tissue (Boyan B D et al., Clin Plast Surg. 1999, 26(4):629-645; Ishaug S L et al., J Biomed Mater Res, 1997, 36(1):17-28).
However, it is known that these polymers can actually be toxic, because they release lactic acid as they degrade, and moreover they may induce an inflammatory response thus inhibiting the bone regeneration process.
Ceramics too, like hydroxyapatite, tribasic calcium phosphate, and calcium sulphate, have been widely used in bone regeneration because they are biocompatible and have osteoinductive potential.
Also known is the use of proteins (and other molecules) of the extracellular matrix for the formation of porous and/or fibrous structures (such as collagen, laminin, fibronectin, and hyaluronic acid) that enhance osteoblast migration and differentiation because they can be loaded with osteoinductive trophic factors.
As we have already said, the main trophic, osteoinductive factors are BMP and TGF, and they are able to direct stem cells to differentiate into osteoblasts and subsequently osteocytes.
BMP was first isolated from demineralised bone specimens. Indeed, as early as 1965, it was demonstrated that such demineralised matrices (DM) induced the formation of new bone structures (Urist M R, Science 150: 893, 1965).
Further studies subsequently clarified the role of BMP in the repair/formation of bone tissue.
In 1990, clinical trials on the fusion of vertebrae using various types of carrier containing BMP, in comparison with autologous bone grafts (Boden S D et al., Spine, 2000, 25(3):376-381), showed that the protein determined a high fusion rate with consequent increase in the mechanical stability of the fused vertebrae. The process of producing DM consists in pulverising bone samples into particles with a diameter of 70-450 μm prior to partially or totally demineralising them with 0.5 N of HCL.
This process enables the total or partial maintenance of the organic component of the bone tissue, ensuring also the integrity of the proteins (and therefore of the gowth factors) contained therein.
The ability of bone to regenerate when damaged is due to certain peculiar features:                osteogenic capacity;        osteoinductive capacity;        osteoconductive capacity.        
In spinal surgery, this last property is linked with the presence of a scaffold fixed to the structures to be fused, allowing the migration and distribution of both bone progenitor and vascular cells within its structure.
It is known that the best examples of scaffolds are autologous and/or allogenic bone grafts, demineralised bone matrices, ceramics, bone substitutes constituted by molecules of extracellular matrix (such as collagen and glycosaminoglycans), even though it is obvious that only bone grafts and DM can be defined as osteoinductive, because of the intrinsic presence of differentiating factors. The osteogenic and osteo-conductive potential of the scaffolds listed above can be considerably increased by introducing bone-progenitor cells, possibly derived from:                whole bone marrow;        bone marrow treated for the preparation of purified mesenchymal cells (possibly also expanded in vitro);        bone marrow treated for the preparation of mesenchymal cells expanded in vitro and also partially differentiated towards the induction of osteoblasts using osteoinductive factors such as TGF β1 and BMP.        
Various scientific experiments have already demonstrated the validity of the use of autologous and/or allogenic mesenchymal cells loaded into different carriers/scaffolds or into matrices constituted by molecules of the extracellular matrix, or by synthetic and/or semisynthetic polymers, or into ceramics, possibly associated with differentiating factors in the regeneration/formation of new bone tissue (Horwitz E M et al., Nat Med, 1999, 5(3):309-313; Gregory A H et al., Neurosurg Focus, 2001, 10(4):1-5; Pilitsis J G et al., Neurosurg Focus, 2002, 13(6):1-6).
For the above reasons, there are many known types of bone graft for use in spinal orthopaedic, neuro-maxillofacial and dental surgery, in orthopaedic surgery to the shoulder, hand and foot, in oncological surgery and in all those pathologies requiring the regeneration/formation of new bone tissue (hence also in pathologies where the fusion of two adjacent bones is indispensable), such as in the following examples:                demineralised and freeze-dried bone powder in a simple mixture with glycerol; (U.S. Pat. No. 5,073,373);        hydrogels constituted by hyaluronic acid or chitosan, cross-linked and with a high molecular weight, containing particles of demineralised bone and possibly also BMP (U.S. Pat. No. 6,326,018);        porous, biodegradable, three-dimensional matrices, containing a mesh of fibres constituted by mineralised polymers such as collagen immersed in other polymers (such as cellulose, hyaluronic acid, chitosan and others of synthetic origin), and possibly also containing bone marrow cells (U.S. Pat. No. 5,776,193);        grafts constituted by porous, composite scaffolds, containing hydrophilic materials such as collagen, glycosaminoglycans and other synthetic and/or semisynthetic polymers, as vehicles for proteins such as BMP (EP 0784985);        bone substitutes constituted by an organic matrix of demineralised bone, subsequently treated with glycosaminoglycans, containing bone-inductive factors for bone regeneration (U.S. Pat. No. 6,165,487);        artificial bone substitutes mainly formed by collagen in matrices of calcium sulphate (U.S. Pat. No. 5,425,769);        bone substitutes composed of demineralised bone matrices and poloxamer as carrier;        porous synthetic matrices containing polymers such as collagen and glycosaminoglycans cross-linked ex vivo, also containing osteogenic proteins and setting agents such as methylcellulose (U.S. Pat. No. 6,468,308);        bone grafts constituted by particles of demineralised bone, in a carrier containing hyaluronic acid together with cellular material and possibly also trophic factors (US 2002/0197242);        devices constituted by three-dimensional macrostructures of D,D,L,L-polylactic acid with microstructures of hyaluronic acid as a carrier for BMP (J. Biomed. Matter. Res. 1999, Spring 48(1):95);        bone grafts formed by porous, three-dimensional matrices similar in structure to sponges, constituted by particles of demineralised bone (WO 02/05750).        